In today's society, many devices include a controller that generates a control signal that controls movement and/or operation of another component of a device. One example of such a device is a Hard Disk Drive (HDD). In a typical HDD, data is stored on a circular disk. Heads for reading and/writing are located on an arm that is positioned over the disk. A track servo system moves the arm over the disk to position the heads over a particular portion of the disk for reading and/or writing of data to that portion of the disk as the disk is rotated by a motor. A controller generates control signals that are transmitted to the servo system to position the arm over the disk to read and/or write desired data.
As technology has advanced, HDDs have become smaller and are included in more devices. As the HDDs have become smaller, a problem encountered is that the servo systems in HDDs are required to hold read/write heads to very small off-track errors to support the increasing track density of disks. Tracking errors can be induced due to many effects including disk and bearing run-out; servo-track-writer induced irregularities; electronic noise; spindle and actuator resonances; and external shock and vibration excitations. The tracking errors cause noise in the control signal applied to the servo system by the controller. The controller monitors the signal applied to the plant to correct the signal to account for the noise added by these tracking errors.
Typically, the tracking errors induced by disk and bearing run-out; servo-track-writer induced irregularities; electronic noise; and spindle and actuator resonances cause high frequency noise in the control signal. The controller can adjust the control signal applied to remove this noise. However, tracking errors from external shock and vibration excitations are typically low frequency noise in the control signals. In the past, observer circuitry had to be added to the control signal circuit to reduce the low frequency error signals. The observer circuitry estimates the low frequency noise and tries to attenuate the low frequency noise from the control signal. The observer circuitry often requires a lot of real estate in the control circuitry adding to the cost and complexity of the design. An example of prior art observer circuitry is provided in U.S. Pat. No. 7,319,570 issued in the name of Jia et al. entitled “Random Vibration and Shock Compensator Using a Disturbance Observer”.
The low frequency noise is a particular problem in small form factor HDDs used in portable environments where there is an increased exposure to continuous shocks and random vibration due to daily activities like jogging, cycling, etc. Portable devices, such as MP3 players, require a smooth flow of music without interruption during jogging. External shocks to the portable device may cause the read/write head in the disk drive to move off-track due to mechanical imbalance of the actuator. Hence, the inability of the HDD to load information from drive to memory buffer. If the drive cannot recover fast enough from shocks, the system will hang. The hanging causes a portable device, such as a MP3 player, to stop operating. To make the disk drive embedded in the portable device suitable for these types of uses, the read/write head must be able to recover from shocks quickly before the next shock arrives. Thus, those skilled in the art are constantly striving to provide a system to remove the low frequency noise from control signals induced by these types of uses while reducing the amount of real estate or space needed in the control signal circuitry.